The number of surface mount electronic components using ceramic for various electronic element packages increases. Most of them are structured so that an electronic element is contained in a ceramic package in which a concave portion is formed and so that an opening portion of the ceramic package is airtight sealed by a metal flat cap (hereinafter “sealing board”).
FIG. 6 is a cross-sectional view of a crystal oscillator sealed by a ceramic package. Reference numeral 1 denotes a surface mount crystal oscillator, and reference numeral 4 denotes a ceramic substrate constituting a package of the surface mount crystal oscillator 1. Reference numeral 3 denotes a flat sealing board, which covers up the ceramic substrate 4 constituting the package of the surface mount crystal oscillator 1 and which is normally made of Kovar (alloy of iron, nickel, and cobalt). Reference numeral 2 denotes a crystal piece, which is contained in an inner space 11 restricted by the ceramic substrate 4 and the sealing board 3.
A wiring layer 6 is formed on an upper surface of the ceramic substrate 4 so as to prevent the wiring layer 6 from becoming conductive to the sealing board 3, and is made electrically conductive to a wiring layer that is a terminal electrode 8 formed on a lower surface of the ceramic substrate 4 by an internal wiring 5. One end of the crystal piece 2 is bonded onto the wiring layer 6 via a conductor paste 7. Furthermore, a metallization layer 9 bonding the sealing board 3 is formed on the upper surface of the ceramic substrate 4. The ceramic substrate 4 is bonded to the sealing board 3 by forming a brazing filler metal 10 serving as a bonding layer on the metallization layer 9 or a sealing surface side of the sealing board 3 and fusion bonding the ceramic substrate 4 to the sealing board 3.
FIGS. 7(a) and 7(b) are a front view and a cross-sectional view showing the sealing board, on which the brazing filler metal is formed, as viewed from a brazing filler metal side, respectively. The brazing filler metal 10 serving as the bonding layer is fusion bonded to a fusion-bonding portion of the sealing board 3 which is a bonded portion to the ceramic package, thereby forming a sealing board with the brazing filler metal for the package. As a material of the brazing filler metal 10, a gold-tin alloy, a lead-tin solder, a tin-copper alloy, a tin-silver alloy or the like is used according to a purpose of the brazing filler metal 10.
As a method of producing a sealing board, there is disclosed a method including pressing a thin brazing filler metal foil into a ring shape (a shape corresponding to the fusion-bonding portion), combining the ring-shaped thin brazing filler metal foil with a sealing board that has been subjected to a surface treatment (which is generally Ni/Au plating; however, which can be Ag plating or Sn plating) for ensuring adhesiveness to a brazing filler metal, conducting a heat treatment at a temperature equal to or higher than a melting point of the brazing filler metal, and fusion bonding the ring-shaped brazing filler metal onto one surface of the sealing board (see, for example, Background Art Section of Japanese Patent Application Laid-Open No. 2002-9186).
There is also disclosed a method of forming a brazing filler metal by plating or printing (see, for example, Japanese Patent Application Laid-Open Nos. 2003-163298 and 2003-163299) The sealing board is molded into a form in which many sealing boards are arrayed and connected via sealing board connectors, and dealt with in a state in which the sealing boards are positioned and fixed, similarly to the method disclosed in the Japanese Patent Application Laid-Open No. 2002-9186.
FIG. 8 is a top view of a metal board formed by arraying many sealing boards on a metal substrate in a matrix. Positioning holes 24 and many sealing boards 21 are formed on the metal substrate 20 by either pressing or etching. The respective sealing boards 21 are connected to an outer peripheral frame 23 by connection bars 22. A brazing filler metal 25 is fusion bonded to each sealing board 21 by directly forming the brazing filler metal 25 on a brazing filler metal fusion-bonding portion by plating or printing, or by punching out a brazing filler metal foil and setting the punched foil onto each sealing board 21. Thereafter, a group of the sealing boards is separated into individual sealing boards each with the brazing filler metal. In case of forming the brazing filler metal after producing the sealing board, the brazing filler metal is fusion bonded to the sealing board by setting the brazing filler metal and the sealing board on a tool.
The sealing board as stated above has the following problems. It is difficult to manage a thickness and a width of the sealing board required for sealing in a state in which the brazing filler metal is fusion bonded to the sealing board. To ensure high adhesiveness between the sealing board and the brazing filler metal, it is necessary to heat the brazing filler metal for long time or at high temperature when the brazing filler metal is fusion bonded to the sealing board. As a result, the brazing filler metal is wetted and spread up to unnecessary portions. Due to this, the brazing filler metal has irregular thicknesses and a portion in which the brazing filler metal is thin is formed, resulting in a sealing defect.
Moreover, if airtightness of an interior of a package is an important factor for characteristics for a case of a crystal oscillator, discharge of air bubbles present in the brazing filler metal into the package during bonding of the ceramic package to the sealing board causes deterioration in characteristics. To sufficiently defoaming the air bubbles present in the brazing filler metal, it is necessary to heat the brazing filler metal for long time or at high temperature when the brazing filler metal is fusion bonded to the sealing board. In this case, the problem of wetting and spreading the brazing filler metal similarly occurs. Particularly if the brazing filler metal is fusion bonded to the sealing board either by a brazing filler metal paste used for printing or by plating, it is necessary to heat the brazing filler metal for long time. To avoid the problems, it is necessary to use an expensive brazing filler metal more than necessary.
Furthermore, there is disclosed a method including plating only a brazing filler metal surface of a sealing board with gold and applying a brazing filler metal onto the gold-plated surface (see, for example, Japanese Patent Application Laid-Open No. 2004-186428). However, this method is accompanied with complicated steps and unavoidably confronted with cost increase. Further, a method including forming a roughened surface by pressing, honing, and etching, and providing the roughened surface as a brazing filler metal flow-preventing band is proposed. However, development of many technical elements is necessary to carry out the method and the method is, therefore, impractical.